Lithographic inks contain vehicles which give the formulations suitable properties for coating the image areas of a printing plate. In a lithographic process, the image and non-image areas are created on thin metal printing plates by a combination of photo-mechanical and photo-chemical processes which make the image area ink receptive and the non-image area water receptive. In the printing process the plate is wet first with water and then immediately thereafter with ink thereby creating the desired image in the ink receptive area of the plate. The image areas are oleophilic and hydrophobic whereas the non-image areas are hydrophilic and oleophobic. Accordingly, the image areas accept ink while the non-image areas are wet with water as a result of the physiochemical principle that oil and water do not mix. When using an ideal ink formulation, the ink only adheres to the image areas of the plate during the inking process.
After coating the image areas of the plate with ink, the plate then transfers ink from the image areas to a blanket or intermediate roller, which, in turn, transfers ink to a substrate or media to be printed. This method of printing is used in most commercial lithographic printing processes and is referred to as "offset" printing.
During the printing operation the ink and water come into contact with one another causing some degree of emulsification of the ink. The degree of emulsification of the ink in a lithographic printing system is important to the proper performance of the system. Too much water emulsification results in poor adhesion of the ink to the image areas whereas too little water emulsification results in poor image quality. Accordingly, the water balance in the ink formulation is critical to maintaining suitable printing operations.
High molecular weight phenolic or maleic modified rosin esters are commonly used in lithographic ink vehicles as a primary resin for improving the gel structure of the ink in order to improve the print quality and reduce misting of the finished ink during printing. It is common to include with the modified resins a secondary or co-resin to further modify the ink properties such as the ink transfer and ink oil solubility of the primary resin component.
Hydrocarbon-based resins have typically been used as suitable secondary or co-resins for improving the solubility of the primary resins in the ink oils. Hydrocarbon-based resins generally exhibit low water take up rates and thus have little effect on water emulsification of the ink. However, hydrocarbon-based co-resins generally possess poor compatibility with alkyd resins, vegetable oils and modified vegetable oils which are normally used in ink vehicle formulations. Hydrocarbon based resins also present increased environmental concerns. Such environmental concerns have prompted the use of base-catalyzed rosin esters as substitute secondary or co-resins.
Base-catalyzed rosin esters have comparable aliphatic oil dilutability and viscosity properties to the conventionally used hydrocarbon-based resins, and pose fewer environmental concerns. However, based-catalyzed rosin esters generally possess higher water pick up rates than hydrocarbon-based resins and tend to destabilize the water balance properties of ink formulations in printing applications.
Accordingly, it is an object of the invention to provide an improved vehicle for ink formulations.
Another object of the invention is to provide a method for making an ink vehicle for a lithographic ink formulation to enable production of ink having a relatively low water pick up rate.
Yet another object of the invention is to provide an environmentally friendly ink vehicle for a lithographic ink formulation.
Another object of the invention is to provide a secondary resin component for ink vehicle formulations which possesses substantial compatibility with alkyd resins and vegetable oils and which improves the solubility of a primary resin component in ink solvents.